Frequency adjustment (trimming) is typically accomplished by a crystal oscillator circuit which usually consists of a crystal, resistance-capacitance feedback circuit, and an on-chip amplifier. Due to significant variations in the crystal resonant frequency and component tolerances, some form of frequency adjustment is usually required to achieve the level of accuracy required in many real-time clock applications. For example, a crystal may have a frequency tolerance of 20 ppm (parts per million) which, at 32.768 Khz, can result in a time deviation of about 2 seconds per day. In some earlier real-time clock designs, frequency trimming has been achieved by varying one or more trimming capacitors during the final board assembly and testing procedure. This method of trimming has several inherent disadvantages, for example, the trimming capacitor must be easily accessible for adjustment, a variable capacitor is significantly larger than a fixed capacitor having the same capacitance value, and capacitor trimming prohibits integrating the capacitor on-chip. In addition, typically after the trimming operation has been completed, the variable capacitor is usually covered with epoxy so that inadvertent capacitance changes do not occur. This approach not only reduces reliability, but also makes future adjustment of the trimming capacitor much more difficult.
Because of the foregoing disadvantages associated with prior art frequency trimming apparatus and methods, it has become desirable to develop apparatus for digitally trimming the frequency of a real-time clock.